Naegele's Rule: How Your Due Date Gets Calculated From One Date

A due date sounds like a fact: "the baby is due March 12." In reality it's an estimate built from a single number — the first day of your last menstrual period (LMP) — run through a 250-year-old formula. Understanding what that formula actually assumes explains both why the due date is useful and why almost no one delivers on it.

The 280-day rule

Naegele's rule, the standard used across most pregnancy calculators and many clinical intake forms, adds 280 days — 40 weeks — to the first day of the LMP. That's not 40 weeks from conception; it's 40 weeks from the start of the cycle in which conception happened, because the LMP is the one date most people can state with confidence, while the actual date of ovulation or fertilization usually isn't known.

The formula folds in an assumption: a 28-day cycle with ovulation on day 14. Add roughly 266 days of gestation from conception to that day-14 ovulation point, and you land on 280 days from the LMP. It's a clean number built on an average cycle — which is exactly where it starts to drift for anyone whose cycle isn't 28 days.

Where the estimate gets less accurate

• Irregular or longer cycles shift the real ovulation date away from day 14, so the LMP-based estimate can be off by several days to over a week in either direction.
• Uncertain LMP — a common situation for anyone with irregular periods, recent hormonal birth control use, or breastfeeding-related cycle changes — removes the one input the whole calculation depends on.
• Early ultrasound dating, typically performed in the first trimester, measures the embryo directly and is considered more accurate than LMP-based dating alone, especially when the two disagree by more than about a week. Clinicians will often use the ultrasound date over the LMP date once one is available.

The trimester markers

The same 280-day count is what the Pregnancy Milestone Calculator uses to project the trimester boundaries, not just the due date. From the LMP: the first trimester is generally considered to end around 12 weeks, and the second trimester around 27 weeks, leaving the third trimester to run to the 40-week due date. Those markers are commonly used to time screening windows and appointment schedules — another reason a single, consistent LMP date matters more than trying to guess a more "precise" conception date.

Because every marker — both trimester boundaries and the due date itself — derives from the same single LMP input, they move together. If your LMP date turns out to be off by a week once an ultrasound comes back, all three markers shift by roughly that same week, not just the due date at the end. That's a useful thing to know if you're trying to time something around the end of the first trimester specifically (many people wait to announce a pregnancy at that point) rather than around the due date itself — the same uncertainty band applies to both.

Why the due date moves — and why that's normal

Only a small share of babies arrive on their exact calculated due date; a full-term range spanning a couple of weeks on either side is the clinical norm, not a deviation from it. The due date is best treated as the center of a range rather than a deadline. Providers routinely revise it as better information becomes available — a first-trimester ultrasound is the most common reason an initial LMP-based date shifts by a few days.

What if you don't know your exact LMP date?

This is more common than it sounds — irregular cycles, recently stopping hormonal birth control, or simply not tracking closely all make the first day of the last period hard to pin down precisely. If you only have an approximate week, run the calculator with your best estimate and treat the result as a rough placeholder rather than a firm date; flag the uncertainty to your provider at the first prenatal visit, since an early ultrasound is specifically useful for resolving exactly this situation. Guessing a few days off in either direction has limited practical consequence at this stage — the bigger risk is treating an uncertain LMP-based date as more precise than it is once travel or leave plans get built around it.

Why this differs from a pregnancy-tracking app

Most consumer pregnancy apps layer weekly symptom content, fetal size comparisons, and kick-counters on top of the same underlying LMP-based due-date math. The date calculation itself is not proprietary or different between a dedicated app and a plain date calculator — both are running some version of Naegele's rule, or a close variant of it, from the same starting input. If the two disagree on a due date by more than a day, one of them is likely using a slightly different LMP date, a different assumed cycle length, or has been manually adjusted based on an ultrasound — not a difference in method.

Use it for planning, not diagnosis

The Pregnancy Milestone Calculator takes your LMP date and instantly returns the Naegele's-rule due date along with the two trimester boundaries, so you can plan around milestones without doing the week-counting by hand. It's built for scheduling and planning purposes — work leave, travel windows, nursery deadlines — not as medical guidance. Your actual due date, and any adjustment to it, should come from your prenatal care provider, who has access to ultrasound dating and your full history that a date calculator does not.